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Fluid
Fluid Mechanics obviously encompasses a vast array of problems that may from the study of blrrd flow in the capillaries to flow of crude oil across alaska through 800-mile-long,4-ft-diameter pipe. The Principles of this field are needed to explain why airplanes are made streamlined with smooth surface for the most efficient flight, whereas golf balls are made rough surfaces to increase their efficiency. Created by Sang-in Park __TOC__ Definition of Fluid A''' fluid is defined as a substance that continuously deforms (flows) when acted on by a shearing stress of any magnitude. All gases are fluids, but not all liquids are fluids. Fluids are a subset of the phases of matter and include liquids, gases, plasmas and, to some extent, plastic solids. *Gas and Liquid :The molescules of a gas are much farther apart than those of a liquid. Hence a gas is very compressible and when all external pressure is removed, it tends to expand indefinitely. A gas therefore in equilibrium only when it is completely enclosed. A liquid is relatively imcompressible, and if all presure is removed the sohesion between molecules hold them together, so that the liquid does not expand indefinitely. Therefore a liquid may have a free surface. *Fluid as a continuum Distinction Between a Solid and a Fluid In Macroscopic View The fluid deforms continously under the influence of a shear force - i.e., the fluid particles continuously change their position relative to one another when subjected to a shear force. On the other hand, solid may resist a shear force when at rest and if they deform, they do not continue to deform indefinitely. Fig.1 shows the behavior of a solid and a fluid under influence of shear force. In case of a solid the deformation is samll and the angular deformation is not a continous function of time, but in the case of a fluid the deformation is large and the angular deformation is continous function of time. In Microscopic View The molecules of a solid are usually closer much together than those of a fluid. The attractive forces between the molecules is inversely proportional to the square of the distance between them Since the molecules of a solid are located close to one another, the forces are larege and, therefore, they offer a great resistance to any external force. In fluid the force of attraction between molecules are only large enough to hold them together to give a definite shape(liquid) or neglectable (gas). Therefore, when an external force is applied to a fluid, its molecules get rearranged continously until the force is removed and do not back to their original positions after the force removed. *Example :A lump of tar may look like a solid. When placed on the ground, it does not spread quickly as water does. However, it does strart to deform as soon as placed on the ground. After sufficient time, perhaps a few days, it will spread just like any other fluid. Classification of Fluid Compressible vs Incompressible Fluid the specific weight is equal to the product of fluid density and acceleration of gravity. Thus changes in specific weight are caused by a change of density or acceleration of gravity. Since in most enginnering application the variation of gravity is neglectible, the density is major factor. the compressible fluid is the fluid that its density can change with changes in pressure or temperature. However, The density of incompressible fluid is constant Ideal(Invicid) vs Real(Viscous) Fluid The ideal fluid is a fluid in which there is no friction; it is inviscid(its viscosity is zero). Thus the intenal forces at nay section within it are normal to the section, even during motion. The r'''eal fluid is Tangential or shearing forces always come into being whenever motion relative to a body takes place, thus giving rise to fluid, because these forces opposite the motion of one particle past another. These friction give rise to a fluid property called viscosity. Newtonian vs Non-Newtonian Fluid Sir Isaac Newton showed how stress and the rate of strain are very close to linearly related for many familiar fluids, such as water and air. These Newtonian fluids are modeled by a coefficient called viscosity, which depends on the specific fluid. However, some of the other materials, such as emulsions and slurries and some visco-elastic materials (eg. blood, some polymers), have more complicated non-Newtonian stress-strain behaviours. Newton's Equation of Viscosity Newton's observation from the two plate experiment #Keeping the area A and the distance Y'' constant, the velocity ''U attained by the plate is directly propotional to the applied force F'' #*''F OC U'' #Keeping the velocity ''U and the distance Y'' constant, the force required to move the plate with a velocity ''U, is directly proportional to the area of plate #*''F'' OC A'' #Keepinf the velocity ''U and the distance Y'' constant, the force required is inversly proportional to the distance between the plates #*''F OC (1/''Y'') Newton's equation of viscosity :In the case of two parrallel plates, the lower surface is assumed to be stationary while the upper one in moved parallel to it with velocity V. If the separation distance Y is not too great, the velocity profile is will be linear as Fig. 2. :This relationship can be expressed by following eqaution: : ::: : : :In above equation, the constant is a measure of internal fluid resistance to relative motion between layers. This constant is called viscosity of fluid. The viscosicity is generated by the friction forces in the flowing fluid result from the cohesion and momentum interchange between molecules. Thus, it is dependent on temperature.As Fig. 4 the viscosicities of liquids decrease as the temperature increases while the viscositicies of all gases increase. Non-Newtonian Fluid :A fluid for which the constant of proportionality does not change with rate of deformat ion. :Fig. 4 shows the difference between newtonian and non-newtonian fluid. Non-newtonian fluid has Reference Fluid Mechanics With Engineering Application, Joseph B. Franzini and E. john Finnemore,9th edition,McGraw-Hill. Fundamentals of Fluid Mechanics,Bruce R. Munson. Donald F. Young and Theodore H. Okiishi, 5th edition, John Wiley & sons. Fluid Mechanics, Irfan A. Khan, 1st edition, Holt, Rinehart and Winston. http://en.wikipedia.org/wiki/Fluid_dynamics http://en.wikipedia.org/wiki/Fluid_mechanics Category:Fixed Wing Design Category:Things You Should Know Category:Aerodynamics